Changes in intracellular calcium levels in response to vasoconstrictors in primary aortic vascular smooth muscle cells derived from wild-type and rgs2^–/– mice. (a) Dose-response relationships for P2Y receptor–mediated increases in intracellular calcium levels. Cells were treated with the indicated doses of ATP, an agonist for P2Y receptors. Changes in intracellular calcium levels were measured as changes in fluorescence ratio [FR ± SEM = (emission at 510 nm upon excitation at 340 nm)/(emission at 510 nm upon excitation at 380 nm)] in fura-2–loaded cells isolated from wild-type (n = 15–22 individual cells for each agonist dose; open squares) and rgs2^–/– mice (n = 15–22 individual cells for each agonist dose; filled squares). (b) Rates of intracellular calcium level decay (100% × [FR – FR_final]/[FR_initial – FR_final] ± SEM) after a maximal response had been elicited by treating cells from wild-type (n = 39 individual cells; open squares) and rgs2^–/– mice (n = 46 individual cells; filled squares) with 100 μM ATP. Pairwise comparisons of the FR values of wild-type and rgs2^–/– cells were made at similar agonist concentrations in a or similar times after maximal FR response in b. Statistically significant differences are indicated (*P < 0.001).